JUVENILE WOOD IN CONIFERS

Transcription

1 '..re wen!!i 7" 7 ROOM 1 JUVENILE WOOD IN CONIFERS No September 1957 IIf Hi filirrnitinhihrom it lei!! I FOREST PRODUCTS LABORATORY MADISON 5, WISCONSIN UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE In Cooperation with the University of Wisconsin

2 JUVENILE WOOD IN CONIFERS By BENSON H. PAUL, Forest Products Technologist 1 Forest Products Laboratory, Forest Service U. S. Department of Agriculture The term "juvenile wood" is sometimes applied by technologists and foresters to wood with relatively wide growth rings and low density that is formed near the pith of the stem. The term is descriptive of the wood rather than indicative of a specific number of years of growth. Juvenile wood is followed by growth with a progressive reduction in ring width, which consists of a more normal, mature type of wood. The number of years involved in the change may differ in individual trees, and the changes that occur either in ring width or wood density may be large or small. Juvenile wood is usually found in plantation-grown trees or in trees so favorably situated that growth in diameter is rapid from the beginning. In these situations the trees that have developed relatively large crowns support wide-ringed growth until crown closure takes place and competition for space and light gradually restricts crown spread. In work at the Forest Products Laboratory on wood from 11 red pine plantations ranging in age from 14 to 37 years, it was found that wood at the 8- to 12-foot height averaged between and in specific gravity. In these plantations the increase in specific gravity was not entirely consistent with age, although there was a general tendency to that relationship. The plantation with the heaviest wood, 0.372, was 34 years of age and diameter growth of the trees averaged 12.8 rings per radial inch. The plantation producing wood that was lowest in average specific gravity, 0.317, was 23 years old, and diameter growth averaged 5.7 rings per inch. In these plantations the trees with the highest average specific gravity were of slowest growth, particularly in their outer 10 years. 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. Rept. No Agriculture-Madison

3 The 43-year-old red pine plantation at Star Lake, Wis., had low-density wood near the pith, at height levels from 100 to 400 inches in the trees, that averaged about in specific gravity. The increase in specific gravity away from the pith was greater at the lower positions in the trees. The specific gravity was as much as in the outer 6 years at the 100- inch level and decreased upward in the same growth rings to at the 400-inch level. Thus the wood near the ground was approaching the character of mature wood while the rood at the 400-inch level still had the characteristics of juvenile wood (8):- Old-growth timber of this species is reported to average 0.44 in specific gravity (4). Lodewick (3), in his work on summerwood percentage relationships of second-growth loblolly and longleaf pines, found that the inner rings were noticeably different from the rings nearer the periphery of the cross section. This departure appeared between the fifth and tenth rings from the pith. These inner rings possessed a smaller percentage of thick-walled summerwood. There was no reason apparent for this in his data. He suggested that it might be explained on the basis of different metabolic conditions in young stems. Turnbull (10), who calculated the specific gravity of pines in South African plantations, showed that an increase in density occurs from the pith outward. He attributed this density increase for these rapidly growing pines as an effect of age, which continued to the end of the 30-year rotation. While Turnbull did not use the term juvenile wood, he might have done so appropriately. He stated that in these short-rotation crops the outer rings were always the heaviest and concluded that "no apprehension need therefore be entertained regarding the production of light wood by maintaining wide espacements during the latter portions of rotations. " This statement undoubtedly applies well to plantations up to 30 years of age. Young southern pines of fairly slow initial growth, however, produced wood much heavier than the more rapidly grown wood (6). It did not shrink excessively lengthwise as is often typical of trees with initial growth rings that are more than 1/4 inch wide (7). In young trees, wide-ringed juvenile wood was associated with more extensive crown development than that of the trees which produced narrow-ringed wood from the beginning. Comparisons of wood produced during successive periods of growth in young longleaf stands showed that trees having initial wood that was low in specific gravity increased in weight remarkably when ring width was retarded from the range of 3 to 5 rings per inch to between 8 and 14 (6). However, in trees that continued at 5 rings per inch, the density of the wood actually decreased. 2 Underlined numbers in parentheses refer to the literature cited at the end of this report. Rept. No

4 In seven older longleaf pine stands ranging in age from 90 to over 300 years, there was a progressive decrease in specific gravity from the center of the trees to the bark in all of the stands. In the specific gravity specimens taken from next to the pith in trees in these older stands, the number of rings per inch averaged from 11 to 20. In figure 1 is presented a diagram (after Trendelenburg (9)) showing a longitudinal section throughout the length of a tree. This diagram depicts a central, funnel-shaped core of low-density wood, the wider portion being at the base of the crown of the tree. This central part of the bole was termed "crown-formed wood" by Trendelenburg because it was formed when the crown was active along the adjacent trunk. As the active foliage of the crown moved upward with the increasing height of the tree, crown-formed wood was produced at progressively greater heights. In the diagram, the lines limiting the crown-formed core cross the growth-increment zones diagonally from near the base of the tree to the crown and the wood exterior to these lines was called "stem-formed wood. " This is wood produced by the cambium along the stem at varying distances from the crown. This stem-formed wood is of higher specific gravity than the crown-formed wood. There is considerable difference in specific gravity from tree base to crown level in growth rings of the same years. The change vertically within a growth ring is so gradual that any fixed point of change from crown-formed to stem-formed wood must be selected arbitrarily. Trendelenburg's crown-formed wood apparently corresponds to wood of the juvenile type. Marts (5) showed that for 10 years after drastically high pruning of opengrown longleaf pine trees, the wood increased greatly in specific gravity as compared with that formed earlier. During the same time, the specific gravity of the stem wood of the newly developed crown showed only a relatively small increase. An abrupt change lower in the tree from wood of the wide-ringed, crown-formed type to narrow-ringed, heavier, stem-formed wood followed the drastic pruning. The increase in density in the lower stem corresponded with a reduction in proportion of springwood and an increase of springwood at the base of the crown. A predominance of springwood is one characteristic of crown-formed wood. Crown-formed wood has other typical structural characteristics and physical attributes besides wide growth rings. Most fibers of crown-formed wood are shorter than those of stem-formed wood. Fibril arrangement in the fiber walls may differ greatly from that in the more narrow-ringed, stem-formed wood, which gives rise to differential shrinkage along the grain when lumber is dried. Thus frequent warping, bowing, and crooking may occur when a timber contains both crown-formed and stem-formed wood. Rept. No

5 Iablokoff (2) attributes the quality of wide-ringed, ligneous material to a relationship between cambium activity and certain hormonal secretions from the buds. This relationship, he says, is influenced by excessive crown volume, which causes a break in equilibrium of cell formation and a resulting disorganization of wood structure. Iablokoff calls his work the establishment of a new hypothesis on the mechanism of wood formation. illustrations of the type of forest growth that does not have juvenile wood are relatively common. When rather narrow-ringed wood of the highest density to be found in a cross section is formed close to the pith, it is not recognized as juvenile wood. In such wood, the characteristics of ring width, percentage of summerwood, and specific gravity are similar to those of wood farther from the pith. In spruce, Hildebrandt (1) found two kinds of age-specific gravity relationships: (1) a regular increasein specific gravity from the first decade on, and (2) an initially high specific gravity that declined to a minimum value, after which there was a generally regular increase. In the second case, the low point in specific gravity was reached at about 30 years, and the initially high specific gravity value was attained again about 40 years later. He attributed individual differences in single trees to variable growing conditions and observed that "a generalization to the effect of increase in specific gravity with age is not possible." Other illustrations of initially higher specific gravity at the centers of pine trees may be found in records of trees that began growth in the virgin forest or under conditions of severe competition. Thus it may be observed that all forests or forest trees do not begin growth under conditions that produce initially wide-ringed wood (fig. 2). When initial growth rings are narrow, as frequently happens in thickly stocked natural reproduction, changes in specific gravity from the pith outward may be negligible. Then the initial narrow-ringed wood is not regarded as juvenile wood. Actually, this wood may have most of the properties of mature wood. It may be assumed, for the present, that juvenile wood is a product of a vigorously growing crown, which supplies the tree, and particularly that portion of the stem nearer the crown with an abundance of growth substances. This is seen in the central portion of the cross section in figure 2, right. Under severely crowded conditions, young trees do not have a chance to develop such relatively large-sized crowns, and therefore there is no excess of growth substances available to form a preponderance of springwood. While total growth of summerwood is also small, the proportions of springwood to summerwood are likely to be more nearly equal. Because of sharp competition among trees that originate in young thickets, lower branches are killed Rept. No

6 quickly and crown activity is restricted to a higher position on the stem sooner than in trees spaced farther apart. Thus it may be assumed that variations in wood structure from the pith outward are, in the main, reflections of varying tendencies in crown development during the period of the tree's most vigorous height growth. In some stands, changes in crown development and wood density proceed simultaneously and are so similar that characteristics of the wood have been attributed to age. Time might be a more appropriate term than age, because, even though changes are progressive with tree development, they are not necessarily coincident with any paiticular stage in the tree's life. Under variable environmental conditions, both the size and the physiological functioning of the crown may fluctuate as the factors of growth are modified. In turn, the results of photosynthesis also contribute to the pattern of growth and wood structure. It appears, then, that variations in wood structure are caused by a combination of photosynthetic, assimilative, and distributive processes beginning in the foliage of tree crowns. Detailed research on the physiological functions of trees and the limitations of site and environment is therefore needed to find explanations for the phenomena that bring about the formation of juvenile wood. Rept. No

7 Literature Cited (1) HILDEBRANDT, GERD Untersuchungen an Fichtenbestanden fiber Zwachs und Ertrag reiner Holzsubstanz. Deutscher Verlag der Wissenschaften, Berlin. 133 pp., illus. (2) IABLOKOFF, A:kh Action des facteurs ecologique sur la structure et les proprietes mechanique du bois epicea (Picea excelsa Link). Annales d 1 Ecole National des eaux et for ets et de la station de researches et experiences, Tome XIV, fasc. 2, Nancy, France. (3) LODE WICK, J. E Some Summerwood Percentage Relationships in the Southern Pines. Jour. Agr. Research Vol. 46, No. 6, pp , illus. (4) MARKWARDT, L. J. and WILSON, T. R. C Strength and Related Properties of Woods Grown in the United States. U. S. Dept. Agr. Tech. Bul. 479, 99 pp., illus. (5) MARTS, R Effect of Crown Reduction on Taper and Density in Longleaf Pine. South. Lbrmn., Vol. 179, No. 2249, pp , illus. (6) PAUL, BENSON H The Application of Silviculture in Controlling the Specific Gravity of Wood. U. S. D. A. Tech. Bul. 168, pp. 20, illus. (7) (8) Reducing Bowing and Crooking of Lumber Cut from Secondgrowth Southern Yellow Pine. South. Lbrmn., Vol. 156, No. 1962, pp , illus Growth and Specific Gravity Responses in a Thinned Red Pine Plantation. Jour. of Forestry Vol. 55, pp , illus. (9) TRENDELENBURG, R Schwankungen des Raumgewichts wichtiger Nadelholzer nach Wuchsgebiet, Standort und Einzelstamm. Zeitschrift des Vereines deutscher Ingeniere Band 79, Nr. 4, s 85-89, 26. (10) TURNBULL, JOHN M Variations in the Strength of Pine Timbers. South African Journal of Science Vol. XXXIII, pp , illus. (March) Rept. No

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9 Figure 2. --Cross sections of longleaf pine. Left, mature wood at the center of a young tree. Right, a young tree with wide ringed juvenile wood at center. Z M